Method for loading a fibrous stock suspension and arrangement to implement the method.

ABSTRACT

Method for loading a fibrous stock suspension which contains cellulose fibers with calcium carbonate, comprising the following process steps: Adding calcium hydroxide in liquid or dry form, or calcium oxide into the fibrous stock suspension, Adding of gaseous carbon dioxide into the fibrous stock suspension, Precipitation of calcium carbonate through the carbon dioxide and Refining of the fibrous stock suspension during the loading process and washing of the fibrous stock suspension after the crystallizing process and/or the refining process and/or during the refining process and/or after the refining process.

The invention relates to a method for loading a fibrous stock suspensionwhich contains cellulose fibers with calcium carbonate.

When a fibrous stock suspension is processed according to the fiberloading technology calcium carbonate is precipitated. This process hasbeen described already, for example in DE 10 13 998 A1.

It is the objective of the current invention to create additionalmethods for the production of a fibrous stock which is loaded withcalcium carbonate.

In accordance with the current invention this objective is met by amethod comprising the following process steps:

-   -   Adding calcium hydroxide in liquid or dry form, or calcium oxide        into the fibrous stock suspension,    -   Adding of gaseous carbon dioxide into the fibrous stock        suspension,    -   Precipitation of calcium carbonate through the carbon dioxide        and    -   Refining of the fibrous stock suspension during the loading        process and washing of the fibrous stock suspension after the        crystallizing process and/or the refining process and/or during        the refining process and/or after the refining process.

Alternatively, this objective is met by a method comprising thefollowing process steps:

-   -   Adding calcium hydroxide in liquid or dry form, or calcium oxide        into the fibrous stock suspension,    -   Adding of gaseous carbon dioxide into the fibrous stock        suspension, and    -   Precipitation of calcium carbonate through the carbon dioxide,    -   Washing of the fibrous stock suspension prior to feeding the        fibrous stock suspension into a headbox chest that is located        downstream in flow direction of the fibrous stock suspension        and/or into a machine for further processing of the fibrous        stock suspension. The fiber loading technology may be applied        before or after the refining process, depending upon specific        requirements upon the end product.

The current invention describes a method for the production of fiberloaded precipitated calcium carbonate (FLPCC), especially for theproduction of chemical pulp or utilization of chemical pulp in paperproduction. The fiber raw material that is to be loaded may—forexample—be produced from recycled paper, DIP (deinked paper), secondaryfiber stock, bleached or unbleached pulp, mechanical pulp of any type,bleached or unbleached sulfate pulp, broke, linen, cotton, and/or hempfibers (predominantly used for cigarette paper) and/or any other paperraw material that can be utilized on a paper machine.

The inventive method can be utilized irrespective of whether or not theend product contains a filler that was produced by a precipitationprocess in a batch reactor or by a refining process (GCC=ground calciumcarbonate), or whether talcum, silicon, titanium dioxide (TiO₂), etc.are used.

In accordance with the FLPCC process described below the filler materialutilized in other production processes is replaced with the fillermaterial produced according to the fiber loading process technology. Therange of application for the filler produced with the fiber loadingprocess technology extends to the production of paper and toapplications for all paper grades, including packaging papers that havea filler content of between 1 and 60% and/or a white liner having afiller content of between 1 and 60%.

The range of application of the current invention is not limited toutilization of these fillers in paper producing processes; the inventioncan be applied in any paper producing process or related process,including the production of chemical pulp. If a fibrous stock suspensionis treated with the fiber loading technology during paper production, acompletely new product results which offers new and improvedcharacteristics compared to currently known products on the market. Theprocess described below allows for precipitation of the filler (calciumcarbonate)—which is attached evenly distributed exclusively in and onthe fiber stock, especially the paper fiber—directly during stockpreparation in a paper mill.

A combination or an individual application of the inventive designvariations described below results in that only fibrous stock which isloaded with precipitated calcium carbonate is produced, whereby thecalcium carbonate is attached on or in the fibers, or embedded in them;this prevents the formation of loose precipitated calcium carbonate(PCC): An additional wash cycle after the refining process and/or duringthe refining process and/or after the crystallization process in acrystallizer and/or prior to the headbox chest or prior to entry intothe paper machine or by returning the press filtrate to a header tank oranother storage arrangement on the infeed side provides that a constantcalcium hydroxide content is adjusted or regulated in the infeed systemof the fiber loading apparatus. The calcium hydroxide can be addeddirectly in a fiber stock pulper. The press filtrate can be returnedinto the stock pulper system. Calcium hydroxide that is not convertedinto calcium carbonate or which does not attach to the fibers is againreturned to the preceding processes.

Only that filler which is not attached on or in the fibers, in otherwords loose precipitated calcium carbonate is washed out. The fibersthemselves which are provided with filler on the inside and the outsidedo not lose said filler through the wash process and the recirculationof the press filtrate, so that the positive effects of the fiber loadingprocess are maintained.

In addition to the design forms of the current invention which aredescribed in more detail below, we also refer you to the design exampleswhich are described in further detail in U.S. Pat. No. 6,413,365, DE 10107 448 A1 and DE 101 13 998 A1 with which the inventive method can alsobe implemented.

The invention especially comprises a method according to which thefibrous stock suspension is fed into a press arrangement intended tosqueeze out a filtrate. Subsequently, the filtrate is directed back, atleast partially, into a supply-side arrangement for pulping of thefibrous stock suspension, specifically into a reservoir, for example aheader tank. The calcium hydroxide is added at least partially in thearrangement for pulping of the fiber stock. In the complete pulpersystem, specifically in the arrangement for pulping of the fibrousstock, a pH value of between 7 and 12, especially between 8 and 12 ismaintained.

In accordance with the current invention aqueous fibrous stock material,especially aqueous paper stock having a consistency of 0.1 to 20%,preferably between 2 and 8% is used as primary raw material.

Calcium hydroxide in aqueous or in dry form, or calcium oxide are mixedinto the aqueous paper fiber stock in a range of between 0.01 and 60% ofthe existing solids content. A static mixer, a header tank or a pulpersystem are utilized for the mixing process; a pH value in the range ofbetween 7 and 12, preferably between 9 and 12 is applied. The reactivityof the calcium hydroxide is between 0.01 and 180 seconds, preferablybetween 0.05 and 60 seconds. Dilution water is mixed in according topredetermined parameters, to thereby produce an aqueous primary rawmaterial.

Carbon dioxide is added into the moist paper stock suspension accordingto the reaction parameters. Calcium carbonate precipitates here in thecarbon dioxide atmosphere.

At the same time a refining energy in the range between 0.1 and 300kWh/ton dry paper pulp is applied. Compared to conventional processesfor the production of a fibrous stock suspension, the current inventionprovides energy efficient attainment of a higher level of freeness;according to the current invention as much as 50% of refining energy canbe saved. This affects especially all paper grades which undergo arefining process during their production and particularly those thathave a high or very high freeness value, for example FL-cigarette papers(FL=Fiber Loading), FL B&P papers, FL kraft sack papers and FL filterpapers. With these papers which do not require fillers, loose fillerwhich is not deposited on or in the fibers can be removed prior tofeeding the fibrous stock suspension into the headbox chest or prior toentry into the paper machine. The fibers themselves however, are loadedinside and outside with filler, so that the positive effects of thefiber loading technology are maintained.

The high mechanical strengths in the end product which are achievedthrough the high freeness value positively affect the production of allpaper grades, especially FL cigarette papers, FL B&P papers, FL sackpapers and FL filter papers since, due to process based mechanical loadsin the various section of the paper machine, such as the press section,the dryer section or in the section where the web is wound, the producedintermediate product and the end product which is to be produced bear ahigh mechanical load due to utilization of winders, rewinders andconverting machinery. Great mechanical stresses occur especially oncigarette paper during its production. These are often caused byutilization of winders during the production of cigarette paper and bythe low basis weight.

The inventive pre-treatment of the fibrous stock suspension also createsthe provision for improved drying, thereby increasing the efficiencylevel in the production of all paper grades. Residual moisture contentsin the range of 1 to 20% are advantageous.

An additional advantage of the current invention is that greaterbrightness and/or higher optical values with around 15 or more lightnesspoints are achieved on all grades of paper, cardboard or in variouscardboard applications including the white liner on a cardboard layer.

The energy supply during the refining process, specifically the heatvolume and the resulting warming effect are controlled. Crystals invarious forms can be produced, according to the control.

In an additional embodiment of the method the current invention providesthat a static mixer, a refiner, a disperger and/or a fluffer FLPCCreactor are utilized as a reactor, whereby the fibrous stock content,especially the paper content is between 0.01 and 15% in the instance ofa static mixer; at between 2 and 40% in the instance of a refiner and adisperger, especially between 2 and 8% for LC refining and between 20and 35% for HC-refining and between 15 and 60% in the instance of afluffer-FLPCC-reactor.

The current invention also relates to a method according to which anexpenditure of energy of between 0.3 and 8 kWh/t, especially between 0.5and 4 kWh/t is used for the precipitation reaction, especially if norefining process is utilized.

The process temperature is preferably between −15° C. and 120° C.,especially between 20 and 90° C. Preferably rhombohedral, scalenohedronand spherical crystals are formed, whereby the crystals measure between0.05 and 5 μm, especially between 0.3 and 2.5 μm.

Static and/or moving, especially rotating mixing elements may beutilized for the production of a fibrous stock suspension which isloaded with calcium carbonate.

The process is preferably carried out in a pressure range of between 0and 15 bar, especially between 0 and 6 bar. Also, the process is carriedout at an pH value that is preferably between 6 and 10, especiallybetween 6.5 and 9.5. The reaction time is here between 0.01 and 180seconds, especially between 0.05 seconds and 60 seconds.

An additional advantage when utilizing the inventive technology with theabove referenced paper grades consists in that these can also be furtherprocessed in a calendar. Due to the fact that fiber loading particlesare deposited in, around and on the fibers, when utilizing the fiberloading technology blackening is avoided.

Fibrous stock produced with the fiber loading combination processtechnology possesses a superior dewatering characteristic when comparedwith a fibrous stock produced according to conventional methods; theimprovement in the dewatering capacity is between 5 to 100 ml CSF or 0.2to 15° SR, depending upon the required freeness and filler content. Thisfibrous stock possesses a low water retention value of 2 to 25%,depending upon the raw material that is used in production. Compared toconventional fibrous stock the water can be removed quicker from thefibrous stock suspension, and the fibrous stock dries accordinglyfaster. This also has a positive effect on remoistening which as aresult has diminished in the paper production process, and upon theprintability of the produced paper grades.

The current invention relates also to a device for the implementationone of the methods described previously. For this purpose a static mixerin which the fibrous stock suspension is washed is located prior to adewatering screw.

An additional embodiment of the invention provides that fiber stockfiltrate which was yielded in the dewatering screw can be returned via apipe to a header tank or to another upstream device for the preparationof the fibrous stock suspension.

Preferably, an additional static mixer in which the fibrous stocksuspension is washed is installed prior to a crystallizer.

Another advantageous provision is to locate an additional washer forcleaning of the fibrous stock suspension, following the crystallizer.

In another embodiment of the apparatus an additional static mixer inwhich the fibrous stock suspension is mixed with a filtrate and/or acalcium hydroxide suspension is located prior to the crystallizer.

The invention is described in further detail below, with the assistanceof design examples and drawings:

FIG. 1 a first schematic drawing—preparation of a fibrous stocksuspension for application in a machine for the production of a fiberweb and

FIG. 2 a second schematic drawing.

A pipe line system 1 (FIG. 1) that is equipped with control valves 2, 3is provided for a fibrous stock suspension. The control valve 2 islocated in a pipe 4 through which the pipe line system 1 is connectedwith a static mixer 5. Dilution water and/or preferably filtrate that isinterlaced with calcium hydroxide is added to the mixer 5 via a valve 6.A tank 7 or a container for storage of the fibrous stock suspension islocated following the mixer 5, viewed in direction of fiber stock flowdirection. From the tank 7 the fibrous stock suspension is pumped via apump 8 to an additional static mixer 9. Dilution water is also added tothe mixer 9 via a valve 10. Likewise, the inflow of a calcium hydroxidesuspension is controlled through a valve 11 which is located in a line12.

This is supplied by a preparation unit 13 where solid calcium oxide orcalcium hydroxide is added to water. For this purpose the preparationunit 13 is supplied via a line 14 which is equipped with a valve 15 withwater. The suspension that is produced in the preparation unit 13 isdirected through a pump 16 into a line 12.

The fibrous stock suspension to which calcium hydroxide was added flowsfrom the mixer 9 into a line 17 through a valve 18 to a dewatering screw19 where water is removed from the fibrous stock suspension. The watercan, for example be returned through a line 20 to the mixer 5 asdilution water. Alternatively, or additionally the water that wasremoved in the dewatering screw 19 may also be routed to a storage tank21 for the fibrous stock suspension, or it is returned to the mixer 9.Due to the return flow of calcium hydroxide containing water the pHvalue can be increased and adjusted, in all instances in the units thatare located preceding the dewatering screw 19.

In order to equalize the fibrous stock suspension, said suspension isbrought via a line 22 from the dewatering screw 19 to an equalizingscrew 23. This is followed through a line 24 by a container 25(crystallizer). For the purpose of supplying carbon dioxide this isconnected with a carbon dioxide storage container 30 via a line 29 whichis equipped with valves 26, 27 and a pump 28. Carbon dioxide is suppliedfrom this container into the crystallizer 25 in order to produce thedesired precipitation reaction of calcium hydroxide and carbon dioxidefor the formation of calcium carbonate as a filler in the fibers of thefiber stock.

In addition the carbon dioxide container 30 is connected with theequalizing screw 23 via an additional line 31 which is equipped with avalve 32 and which branches off line 29. Carbon dioxide can herewithalso be supplied to the equalizing screw in order to already achieve atleast a partial precipitation there.

Line 29 is also connected via an additional valve 33 with a static mixer34. This serves to add additional carbon dioxide to the fibrous stocksuspension which is flowing from the crystallizer 25 via a line 36 whichis equipped with a valve 35.

The fibrous stock suspension flows from the mixer 34 into a mixing chest37. A filtration unit 38 may be provided between the mixer 34 and themixing chest 37. From the filtration unit 38 the filtrate which has beenenriched with calcium carbonate is returned into the header tank 7 orinto an other downstream unit for the preparation of the dilution wateror the fibrous stock suspension. The mixing chest 37 is equipped with arotor 39 to thoroughly mix the fibrous stock suspension. From the mixer34 the fibrous stock suspension then flows either immediately to a headbox in a paper machine, or will be subjected to additional mechanicalprocessing, for example in a refiner feed chest.

Fibrous stock suspension to which calcium hydroxide has not yet beenadded can be supplied to the mixer 34 through a pipe line system 1 viathe valve 3 and a line 40 in which said valve is installed.

It is further provided that white water or process water which has forexample been recovered in the wire area or the paper machine or, asalready described previously, fibrous stock suspension from thedewatering screw 19 is supplied to the tank 21. Dilution water may forexample be supplied to this tank through a line 41 which is equippedwith a valve 42.

From the container 21 the dilution water which is mixed with processwater flows through a line 43, a pump 44 and a valve 45 to thecrystallizer 25. According to the design of an arrangement depicted inFIG. 1 for loading of the fibrous stock suspension with a filler,especially with calcium carbonate, a multitude of possibilitiestherefore arise to influence the composition of the fibrous stocksuspension that is to be produced in various stages of the productionprocess.

In another design form a fibrous stock suspension is transported for thepurpose of being loaded with calcium carbonate in a device 48 (FIG. 2)in a pipe line system that is equipped with control valves 49, 50. Thecontrol valve 49 is located in a line 51 through which the pipe linesystem is connected with a static mixer 52. This mixer can be suppliedvia a valve 53 with dilution water. Likewise, an additional valve 55which is installed in line 54 controls the supply of a suspension ofcalcium hydroxide. This is supplied by a preparation unit 56 into whichsolid calcium oxide or calcium hydroxide is fed. For this purpose wateris supplied into the preparation unit 56 via a line which is equippedwith a valve 57. The suspension which is produced in the preparationunit 56 is brought into the line 55 via a pump 58.

Diluted suspension which has been treated with calcium hydroxide flowsfrom the mixer 52 into a line 59 which is equipped with a valve 60. Thesuspension is fed immediately from the line 59 into a container 61(Crystallizer). This is connected with a carbon dioxide storage tank 66by means of a line 65 which is equipped with valves 62, 63 and with apump 64 for the purpose of supplying carbon dioxide. Carbon dioxide isfed from said storage tank into the crystallizer 61 in order to createthere the desired precipitation reaction of calcium hydroxide and carbondioxide for the formation of calcium carbonate as a filler in the fibersof the fiber stock. Instead of utilizing a mixer 52 the calciumhydroxide can also be added from a header tank.

The line 65 is connected through an additional valve 67 with a staticmixer 68 which serves to add additional carbon dioxide to the fibrousstock suspension flowing from the crystallizer 61 through a line 70which is equipped with a valve 69.

From the mixer 68 the fibrous stock suspension flows into a blend chest71 which is equipped with a rotor 72 for blending the fibrous stocksuspension. From the blend chest 71 the fibrous stock suspension flowseither immediately to a headbox in a paper machine or is subjected toadditional mechanical processing, for example in a refiner feed chest.

The blend chest 71 can additionally be supplied via the valve 50 and aline 73 with fiber stock suspension which has not been treated withcalcium hydroxide.

In addition, a refiner 74 may be incorporated in the pipe line system,serving the refining of the fiber stock suspension by means of anadditional refining process. Said refiner is supplied with fiber stocksuspension via a line 75 which branches off line 59. From the refiner 74the fibrous stock suspension which has been refined again is broughtthrough a line 76 into the line 70 and from there, as described aboveinto the storage tank.

In addition, a provision can be made that carbon dioxide is supplied tothe refiner 74 from the carbon dioxide storage tank 66 through a line 77branching off line 65, and a static mixer 78 connecting line 77 withline 75.

In this arrangement for the preparation of the fibrous stock suspensionthe container 74 additionally assumes the refining process, therebycreating a simple design for the machine line for fiber stockpreparation. The refining process serves at the same tune as anagitation process in order to deposit the calcium carbonate in thefibers through a shear process.

A filtration unit 79, similar to the filtration unit 38 may be locatedalso between the mixer 68 and the blend chest 71 from where filtratetreated with calcium carbonate is returned into a header tank or intoanother preceding unit for preparation of the dilution water or thefibrous stock suspension.

1-28. (canceled)
 29. A method for loading a fibrous stock suspensioncontaining cellulose fibers with calcium carbonate, comprising the stepsof: adding one of a calcium hydroxide in one of a liquid form and a dryform, and calcium oxide into the fibrous stock suspension; addinggaseous carbon dioxide into the fibrous stock suspension; precipitatingcalcium carbonate through said carbon dioxide; refining the fibrousstock suspension during the loading method; and washing the fibrousstock suspension at least one of after a crystallizing process, afterthe refining step, and during the refining step.
 30. The method of claim29, further including the step of washing the fibrous stock suspensionprior to feeding the fibrous stock suspension into at least one of aheadbox chest that is located downstream in flow direction of thefibrous stock suspension, and a machine for further processing of thefibrous stock suspension.
 31. The method of claim 29, further includingthe steps of feeding the fibrous stock suspension into a pressarrangement to squeeze out a filtrate from the fibrous stock suspension,and at least a partial returning of said filtrate into an arrangementfor pulping the fibrous stock suspension.
 32. The method of claim 31,wherein said filtrate is returned into a supply-side reservoir.
 33. Themethod of claim 32, wherein said supply-side reservoir is a header tank.34. The method of claim 31, wherein said calcium hydroxide is added atleast partially in said arrangement for pulping of the fibrous stocksuspension.
 35. The method of claim 34, further including the step ofmaintaining a pH value of approximately between 7 and 12 at least insaid arrangement for pulping of the fibrous stock suspension.
 36. Themethod of claim 35, wherein said pH value is approximately between 8 and12.
 37. The method of claim 29, further including the step of using anaqueous fibrous stock material having a consistency of approximatelybetween 0.1% and 20% as a primary raw material.
 38. The method of claim37, wherein said consistency is approximately between between 2% and 8%.39. The method of claim 38, wherein said aqueous fibrous stock materialis an aqueous paper stock
 40. The method of claim 37, further includingthe step of mixing said calcium hydroxide into said aqueous fiber stockmaterial whereby said aqueous fiber stock material has a solids contentof approximately between 0.01% and 60%.
 41. The method of claim 40,wherein said aqueous fiber stock material is a paper fiber stock. 42.The method of claim 29, further including the step of mixing saidcalcium hydroxide through one of a static mixer and a header tank. 43.The method of claim 29, further including the step of reacting saidcalcium hydroxide within a range of approximately between 0.01 secondsand 180 seconds.
 44. The method of claim 43, wherein said range isapproximately between 0.05 seconds and 60 seconds.
 45. The method ofclaim 29, further including the step of mixing a dilution water into thefibrous stock suspension one of prior to, during and after at least oneof said adding one of a calcium hydroxide and calcium oxide step andsaid adding gaseous carbon dioxide step.
 46. The method of claim 29,wherein said carbon dioxide is mixed into a moist fibrous stocksuspension.
 47. The method of claim 29, further including the step ofapplying a refining energy in a range of approximately between 0.1 kWper ton dry paper pulp and 300 kW per ton dry paper pulp.
 48. The methodof claim 47, further including the step of controlling an energy supplyby said refining step.
 49. The method of claim 29, further including thestep of utilizing at least one of a static mixer, a refiner, a dispergerand a fluffer FLPCC reactor as a reactor, whereby a fibrous stockcontent is one of approximately between 0.01% and 15% in an instance ofa static mixer; approximately between 2% and 40% in the instance ofeither a refiner and a disperger, and between 15 and 60% in the instanceof a fluffer FLPCC reactor.
 50. The method of claim 49, wherein saidfibrous stock content is a paper content.
 51. The method of claim 49,wherein said instance of a refiner, said fibrous stock content isapproximately between 2% and 8% for LC refining.
 52. The method of claim49, wherein said instance of a refiner, said fibrous stock content isapproximately between 20% and 35% for HC-refining,
 53. The method ofclaim 29, wherein said precipitating step includes an expenditure ofenergy of approximately between 0.3 kWh/t and 8 kWh/t.
 54. The method ofclaim 53, wherein said expenditure of energy is approximately between0.5 kWh/t and 4 kWh/t.
 55. The method of claim 29, wherein a processtemperature is approximately between −15° C. and 120° C.
 56. The methodof claim 55, wherein said process temperature is approximately between20° C. and 90° C.
 57. The method of claim 29, further including the stepof forming rhombohedral crystals, scalenohedron crystals and sphericalcrystals, of said calcium carbonate.
 58. The method of claim 57, whereinsaid crystals measure approximately between 0.05 μm and 5 μm.
 59. Themethod of claim 57, wherein said crystals measure approximately between0.3 μm and 2.5 μm.
 60. The method of claim 29, further including thestep of utilizing at least one of static mixing elements and movingmixing elements.
 61. The method of claim 60, wherein rotating mixingelements are utilized.
 62. The method of claim 29, wherein said methodis carried out in a pressure range of approximately between 0 bar and 15bar.
 63. The method of claim 62, wherein said pressure range isapproximately between 0 bar and 6 bar.
 64. The method of claim 29,wherein said method is carried out at a pH value of between 6 and 10.65. The method of claim 64, wherein said pH value is approximatelybetween 6.5 and 9.5.
 66. The method of claim 29, further including areaction time approximately between 0.01 seconds and 180 seconds. 67.The method of claim 66, wherein said reaction time is approximatelybetween 0.05 seconds and 60 seconds.
 68. A method for loading a fibrousstock suspension containing cellulose fibers with calcium carbonate,comprising the steps of: adding one of a calcium hydroxide in one of aliquid form and a dry form, and calcium oxide into the fibrous stocksuspension; adding gaseous carbon dioxide into the fibrous stocksuspension; precipitating calcium carbonate through said carbon dioxide;washing the fibrous stock suspension prior to feeding the fibrous stocksuspension into at least one of a headbox chest that is locateddownstream in flow direction of the fibrous stock suspension, and amachine for further processing of the fibrous stock suspension.
 69. Themethod of claim 68, wherein said precipitating step includes anexpenditure of energy of approximately between 0.3 kWh/t and 8 kWh/t.70. The method of claim 69, wherein said expenditure of energy isapproximately between 0.5 kWh/t and 4 kWh/t.
 71. An apparatus forloading a fibrous stock suspension containing cellulose fibers withcalcium carbonate, said apparatus comprising: a dewatering screw; and anadditional static mixer prior to said dewatering screw, said additionalstatic mixer being provided in which the fibrous stock suspension isblended with at least one of a filtrate and a calcium hydroxidesuspension.
 72. The apparatus of claim 71, further including a pipeconnected to a header tank which are connected to said dewatering screw,a fibrous stock suspension filtrate being yielded in said dewateringscrew being returned through said pipe to one of said header tank andanother preceding device for fiber stock preparation.
 73. The apparatusof claim 71, further including an additional static mixer preceding acrystallizer, the fibrous stock suspension being washed in saidadditional static mixer.
 74. The apparatus of claim 73, furtherincluding an additional washer after said crystallizer, said additionalwasher for cleansing of the fibrous stock suspension.
 75. The apparatusof claim 73, wherein the fibrous stock suspension can be returned intosaid additional static mixer.